1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic type copying machine or a printer/facsimile, etc.
2. Related Background Art
Conventionally, in an image forming apparatus such as an electrophotographic apparatus, an electrostatic recording apparatus, etc., what has hitherto been a general method of charging the surface of an image bearing member as a member to be charged such as a photosensitive member, a dielectric body, etc., is corona electrical charging that is a non-contact charging method of charging the surface of the image bearing member in a way that makes a corona generated by applying a high voltage to a thin corona discharge wire act on this image bearing member surface.
In recent years, in terms of a low voltage process, a low ozone generation quantity and a lost cost, a mainstream system is a contact charging system in terms of a low voltage process, a low ozone generation quantity and a low cost, in which a charging member such as a roller type charging member, a blade type charging member, etc. is brought into contact with the surface of the image bearing member, and the surface of the image bearing member is charged by applying a voltage to the charging member. In particular, the roller type charging member is capable of performing stable charging over a long period of time (Japanese Examined Patent Publication No. 3-52058).
Only a DC voltage may suffice as the voltage applied to the charging member, however, the charging can be uniformly effected by applying an oscillation voltage to cause discharges toward a plus side and a minus side alternately.
For example, there is a known method exhibiting an effect of uniformizing the charging of a member to be charged by applying an oscillating voltage. The oscillation voltage is obtained by superposing an AC voltage including a peak-to-peak voltage that is twice or larger than a discharge start threshold voltage (charge start voltage) of the member to be charged obtained upon application of a DC voltage thereto and a DC voltage (DC offset bias).
A waveform of the oscillating voltage is not limited to a sine wave and may also be a rectangular wave, a triangular wave and a pulse wave. The oscillating voltage includes a voltage of the rectangular wave formed by periodically switching ON/OFF the DC voltage, and also a voltage having the same output as that of a superposed voltage of the AC voltage and the DC voltage by periodically changing a value of the DC voltage.
As described above, a contact charging system for charging the charging member by applying the oscillating voltage thereto will hereinafter be referred to as an “AC charging system”. Further, a contact charging system for charging by applying only the DC voltage will be referred to as a “DC charging system”.
According to the AC charging system, however, as compared with the DC charging system, a discharge quantity to the image bearing member increases, and hence there might be a case where deterioration of the image bearing member such as a chip-off, etc. is accelerated, and there appears an abnormal image such as an image flow due to a discharging product in a high-temperature, high-humidity environment.
An improvement of this problem entails minimizing the discharging caused toward the plus side and the minus side alternately by applying the voltage at the minimum required.
In fact, however, a relationship between the voltage and the discharge quantity is not invariably fixed but changes depending on a layer thickness of each of a photosensitive member layer of the image bearing member and a dielectric member layer, the charging member, an environmental fluctuation of the air and so forth. In a low-temperature low-humidity environment (L/L), the material is dried with the result that a resistance value increases so that the discharge is unlikely to be caused. Therefore a peak-to-peak voltage having a fixed or greater value is needed for obtaining the uniform charging. In a lowest voltage value with which the uniformity of the charging is obtained in this L/L environment, however, in the case of conducting the charging operation in the high-temperature high-humidity environment (H/H), the material absorbs the humidity, and the resistance value decreases on the contrary, with the result that the charging member causes unnecessary discharging. As a consequence, if the discharge quantity increases, there arise such problems that the image flow and a defocused image are formed, a toner is fused, the chip-off and a shortening of lifetime of the image bearing member due to the deterioration of the surface of the image bearing member are caused, and so on.
In order to restrain the discharging increase and decrease in the discharge quantity due to this environmental fluctuation, an “AC constant current control system” that controls a current value of the flowing AC current by applying the AC voltage to the charging member is proposed in addition to the “AC constant voltage control system” that applies the fixed AC voltage at all times as described above. According to this AC constant current control system, in the L/L environment where the resistance of the material rises, the peak-to-peak voltage value of the AC voltage can be raised. On the contrary, in the H/H environment where the resistance of the material decreases, the peak-to peak voltage value of the AC voltage can be lowered. It is therefore possible to restrain the increase and decrease in the discharge quantity as compared with the AC constant voltage control system.
Herein, the charging member is not necessarily kept in contact with the surface of the image bearing member. The charging member and the image bearing member may be disposed in a non-contact manner in close proximity to each other with an air gap (gap) that is, for example, several tens of μm on condition that just a dischargeable area determined by a gap-to-gap voltage and a compensation Paschen's curve be certainly assured (proximal charging). This proximal charging shall come under a category of the contact charging.
Aiming at a longer lifetime of the image bearing member, however, the AC constant current control system is not yet perfect in terms of fluctuation in resistance value due to ununiformity in manufacturing the charging member and a contamination thereof, fluctuation in electrostatic capacitance of the image bearing member due to endurance thereof, and restraining the increase and decrease in the discharge quantity. Thus, in order to restrain the increase and decrease in the discharge quantity, means for restraining ununiformity in manufacturing the charging member and the environmental fluctuation as well as means for eliminating a deflection in high voltage must be taken. This brings about a rise in cost.
Such being the case, the following contrivances were made (Japanese Patent Application Laid-Open Nos. 2001-166565 and 2001-201920). Namely, a relationship between the voltage and the current is measured by applying the peak-to-peak voltages in a discharge area and an undischarged area to the charging member during a pre-rotation process, image formation, and a sheet space setting process, etc., of the image forming apparatus. Then, the peak-to-peak voltage to be applied to the charging member upon image formation is compensated from the measured value each time and is thus applied. Accordingly, it becomes possible to properly control the voltage and the current applied to the charging member to effect the uniform charging without causing any problems such as the deterioration of the image bearing member, the fusion of the toner, the image flow, etc. in a way that invariably causes a fixed quantity of discharge with no occurrence of an excessive discharge in spite of fluctuation in resistance value of the charging member which appears depending on the environment and the manufacturing process. Further, it becomes also possible to conduct uniform charging irrespective of a contamination of the charging member even during a consecutive image formation, thereby enabling a high image quality and a high definition to be stably maintained over a long period of time.
However, when a peak-to-peak voltage that is twice or smaller than a discharge start voltage Vth at which the charging means starts charging the photosensitive member is applied, in order to measure the current quantity in the undischarged area, a charging potential on the photosensitive member becomes unstable and does not come to a predictable potential state. Especially in the case of using the contact developing for the developing means, even if a power supply to the developing means is suspended, the developer is adhered to the image bearing member as the potential on the photosensitive member attracts. Moreover, in the case of using the two-component developing system, the magnetic carrier in the developer is adhered to the image bearing member as the potential on the photosensitive member attracts, resulting in a cause of a defect in the image.